(a) Single fibre tests
The most direct test involves pulling a partially embedded single fibre out of a block of matrix material (Figure 2.23(a). This test, although simple in principle, is difficult to carry out especially for thin brittle fibres. However if successful, from the resulting tensile stress versus strain plot the shear strength of the interface and the energies of debonding and pull-out may be obtained. These energies are discussed in more detail in Chapter 11 which considers toughening mechanisms.
The interfacial shear strength t may also be evaluated using a speci- men consisting of a block of matrix material with a single, completely embedded short fibre which is accurately aligned longitudinally in the centre of the specimen (Figure 2.23(b). On testing in compression, shear stresses are set up at the ends of the fibres as a consequence of the difference in elastic properties of the fibre and matrix (see Chapter 10) The shear stresses eventually lead to debonding at the fibre ends and ti may be evaluated from the compressive stress oc at which debonding occurs as